Ozone is formed by the interaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. In the earth’s lower atmosphere, ozone is a primary constituent of smog. Approximately 58 percent of VOC emissions come from cars and trucks and off-road vehicles such as lawnmowers. Approximately 85 percent of NOx comes from cars and trucks and off-road vehicles such as construction equipment and trains. Other sources include small stationary sources, biogenic emissions from grass, shrubs, and electric power plants.

Breathing ozone may prompt a variety of health problems such as chest pain, coughing, throat irritation, congestion, increased risk of asthma attacks and other respiratory issues. In 2001, EPA determined that the Maricopa County region had attained the 1-hour ozone standard and the region has currently requested EPA to determine that the area meets 8-hour ozone standard. Stringent requirements are in place to reduce ozone-forming pollutants to meet the standards, as well as a new more-stringent 8-hour ozone standard promulgated by EPA in March 2008.